Human prostate tumors are being grown in tissue culture and also transplanted into the athymic "nude" mutant mouse. The two methods of propagating the cells are being used in order to obtain established cell lines of human prostate which maintain the characteristics of the original tissue including its tumorigenicity and its hormone dependence. Studies are also being carried out on growth of other tumors, both human and animal, in the nude mouse, our particular interest being the identification of the fundamental properties of the cell which make it tumorigenic. We are comparing growth of normal cells, transformed cells, and revertants of transformed cells intending to show the correspondence of some of the in vitro transformed characteristics with the ability to form tumors.